It is known in the prior art to apply ceramic to a metallic substrate to produce a ceramic thermal barrier coating by the thermal, or plasma, spray process. In this technique the ceramic is applied onto a bond coat, for example a MCrAlY bond coat, which has been applied to the metallic substrate.
It is also known in the prior art to apply ceramic to a metallic substrate to produce a ceramic thermal barrier coating by the physical vapour deposition (PVD) process. In this technique the ceramic is applied onto a bond coat, for example a MCrAlY coating with an alumina interface layer, which has been applied to the metallic substrate. The ceramic thermal barrier coatings deposited by the PVD process have benefits over the ceramic thermal barrier coatings deposited by a thermal spray process. The main benefit is improved thermal shock resistance due to the columnar structure of the ceramic thermal barrier coating produced by the PVD process. Other benefits are improved erosion resistance and improved aerothermal performance.
However, despite these advantages, the ceramic thermal barrier coating deposited by the PVD process exhibits a thermal conductivity which is greater than that of a ceramic thermal barrier coating, of the same or similar composition, deposited by the thermal spray process. For example the thermal conductivity of a zirconia-8% yttria ceramic thermal barrier coating deposited by the PVD process is 2.0 W/m/K and the thermal conductivity for the same ceramic thermal barrier coating deposited by the thermal spray process is 0.8-1.0 W/m/K. If all other factors are the same for the two methods of deposition of the ceramic thermal barrier coating, the greater thermal conductivity of the ceramic thermal barrier coating deposited by the PVD process means that a greater thickness of ceramic is required to achieve the equivalent insulating effect when compared to the ceramic thermal barrier coating deposited by the thermal spray process. This is an undesirable property because this necessitates a greater weight of ceramic thermal barrier coating on the metallic components of the gas turbine engine, and this is particularly undesirable for rotating components e.g. turbine blades because the additional weight may limit the temperature of operation due to a corresponding reduction in the creep life of the metallic turbine blade.
A paper entitled "Microlaminate Composites as Thermal Barrier Coatings" by M. C.Radhakrishna, H. J. Doerr, C. V. Deshpandey and R. F. Bunshah was presented at the 15th International Conference on Metallurgical Coatings, at San Diego, USA, 11-15th Apr. 1988 and was subsequently presented in Surface and Coatings Technology, 36 (1988) 143-150. The paper discloses the use of microlaminate composites to reduce the thermal conductivity of physical vapour deposited thermal barrier coatings. The microlaminate composites comprise alternate layers of two different materials, e.g. two different metals, two different ceramics or a metal and a ceramic. The paper specifically describes the use of nickel layers interposed between layers of NiCoCrAlY and titanium layers interposed between layers of CoCrAlY. These microlaminates have thermal conductivities of 7.48 W/m/K for a 480 layer Ni/NiCoCrAlY coating system and 6.76 W/m/K for a 480 layer Ti/CoCrAlY coating system. The paper then suggests that the thermal conductivity of the microlaminate composite may be tailored to obtain thermal conductivity values similar to those of yttrium stabilised zirconia deposited by the thermal spray process by choosing appropriate metal and ceramic microlaminates.
A paper entitled "Sputter-Deposited Multilayered Ceramic/Metal Coatings" by J. W. Patten, M. A. Bayne, D. D. Hayes, R. W. Moss and E. D. McClanahan was presented at the International Conference on Metallurgical Coatings, San Diego, USA, 23-27th Apr. 1979 and was subsequently presented in Thin Solid Films, 64 (1979) 337-343. The paper discloses the use of alternate layers of metal and ceramic. The ceramic layers are to provide erosion, corrosion and possibly thermal insulation, while the metal layers improve mechanical properties. The metal and ceramic layers are sputter deposited. The alternate layers described were nickel interposed between yttria stabilised zirconia and nickel-chromium interposed between yttria stabilised zirconia. The ceramic layers are of the columnar type.
A further disclosure in published European patent application 0366289A entitled "Multi-Layer Wear Resistant Coatings" uses alternate layers of metallic and ceramic materials to provide an erosion and corrosion resistant coating. This discloses layers of titanium and titanium nitride but metal layers of boron, zirconium, hafnium, tantalum or iron may be used and ceramic layers of a nitride, a carbide or an oxide of these metals may be used. The coatings may be applied by sputtering, physical vapour deposition or chemical vapour deposition.
We believe that the concept of inserting metallic layers between layers of ceramic to provide improved performance with regard to the thermal conduction, i.e. to reduce the thermal conductivity and coating ductility of a thermal barrier coating is unsound for the following reasons. It is not thought likely that such an approach will lead to a reduction in the thermal conductivity of the thermal barrier coating and the metal layers will become oxidised leading to instability of the thermal barrier coating.
Furthermore, the use of layers of different materials in a thermal barrier coating necessitates the use of separate sources of metal and ceramic which have to be evaporated, or sputtered, to deposit the alternate layers on the metallic substrate. Alternatively reactive gas sources have to be available to provide reactive gas to react with the evaporated metal to produce the ceramic layers.
A paper entitled "Ceramic Thermal Barrier Coatings with Improved Corrosion Resistance" by J. T. Prater and E. L. Courtright was presented at the 14th International Conference on Metallurgical Coatings, San Diego, USA, 23-27th Mar. 1987 and was subsequently presented in Surface and Coatings Technology, 32 (1987) 389-397. The paper discloses a method for limiting the ingress of corrosive species in physical vapour deposited yttria stabilised zirconia ceramic thermal barrier coatings by inserting dense ceramic sealing layers between the usual columnar ceramic layers. The dense ceramic sealing layers disrupt the columnar ceramic layers to reduce the permeability to liquid and gaseous species and increases the corrosion resistance of the ceramic thermal barrier coating.
The columnar layers were deposited by sputtering and the dense ceramic sealing layers were deposited by sputtering and applying a RF induced DC bias on the substrate. The voltage applied during the period that the bias is applied to the substrate causes renucleation/grain growth within the coating structure and destroys the major, through thickness, columnar boundaries between columnar grains.
However, this paper does not disclose any details of the effect of the dense ceramic sealing layers on the thermal conductivity of the ceramic thermal barrier coating.